Opportunistic infections remain the most important complication of infection with the Human Immunodeficiency Virus (HIV) and the principal cause of death in patients with the Acquired Immune Deficiency Syndrome (AIDS). A large proportion of patients infected with HIV develop severe oropharyngeal and esophageal candidiasis. The predominant infecting species, Candida albicans, can grow as yeast, pseudohyphae and hyphae and the ability to transition between these different cell types is important for virulence. Cell cycle progression, which is controlled by cyclin-dependent kinases, is often intimately connected to the regulation of morphogenesis. Our preliminary data indicate that the two C. albicans B-cyclins (Cyblp and Cyb99p) and a putative transcriptional regulator of cell cycle genes (Fkh2p) are important for distinct aspects of morphogenesis and that Fkh2p regulates the levels of Cyb99 mRNA. We will test four hypotheses: First, that the two C. albicans B-cyclins have distinct roles in cell cycle progression; second, that Fkh2p is a transcriptional regulator of B-cyclin and other cell cycle-regulated genes; third, that C. albicans hyphae have cell cycle dynamics more similar to filamentous fungi than to budding yeast; and fourth, that C. albicans morphogenesis involves specific, fundamental changes in cell cycle dynamics that are also important for virulence. We propose to identify and characterize C. albicans gene products that regulate both cell cycle progression and morphogenesis in order to reveal the molecular mechanisms that underlie the coordination of these processes. Specifically, we will determine the mechanisms by which the B-cyclins and Fkh2p contribute to morphogenesis, cell cycle regulation, and virulence. We will compare wild-type strains and strains lacking Fkh2p or B-cyclins using time-lapse microscopy of fluorescent proteins to follow the dynamics of cell cycle progression. We will use DNA microarrays to follow transcriptional regulation and in vitro aasays of interactions between the pathogen and epithelial tissue or macrophages to study aspects of virulence. Ultimately, fungal-specific gene products that execute essential cell cycle processes may be important new targets for the development of anti-fungal therapies.